This invention relates in general to systems and in particular to a method for controlling such systems.
One example of a common system that needs control is a Heating, Ventilating and Air Conditioning (HVAC) system, which is actually a flow conducting system. A typical HVAC system includes either an expansion valve or a fixed orifice valve that modulates refrigerant flow from a condenser to an evaporator in order to maintain enough suction superheat to prevent any un-evaporated refrigerant liquid from reaching the system compressor. This is done by controlling the mass flow of refrigerant entering the evaporator so that it equals the rate at which it can be completely vaporized in the evaporator by absorption of heat. In the past, capillary tubes and thermostatic expansion valves have been widely used in refrigerating machines as refrigerant flow regulating devices. Now Electrically or Electronically driven Expansion Valves (EEVs) are commonly utilized and permit more advanced control. However, with this type of regulating device, it becomes necessary to choose a control algorithm.
Referring now to FIG. 1, there is shown a typical HVAC system 10 that includes two shell and tube heat exchangers that function as an evaporator 12 and a condenser 14, respectively, and a reciprocating compressor 16. As shown in FIG. 1, the evaporator 12 is used with a flow of water and antifreeze mixture as a secondary refrigerant fluid, whereas the condenser 14 is water cooled. The refrigerant fluid is vaporized inside tubes disposed within the evaporator 12, while its condensation occurs outside the tube bundle in the condenser 14. The refrigerant travels about a closed loop that is labeled 18. The flow of the refrigerant is controlled by an EEV 20. Air to be cooled is forced through the evaporator 12 by a blower 22 connected to an intake vent 24. The cooled air is discharged through a discharge vent 26. Typically, ductwork (not shown) routes the air before and after passage through the evaporation 12.
In the past, the EEV 20 has been an electronic valve controlled by the displacement of a magnet in a magnetic field created by a coil. The displacement of the magnet induced a linear movement of the needle and, consequently, a proportional throttling of the valve. The EEV 20 typically had a precise positioning control loop with a stroke resolution of up to 1:1000 and a positioning time that may be as fast as less than one second. The control signal needed to operate this valve was usually obtained by a package that contained a PID controller and a pressure and temperature sensor. More recently, micro-valve arrays, such as those available from DunAn Microstaq, Inc., of Austin, Tex., have been substituted for the EEVs to achieve energy savings through more precise and rapid control of superheat.
The differences among the micro-valve systems are based upon geometry, actuation mechanisms, membrane material, flow path design and fabrication techniques. Among these differences, the actuation mechanism is the most commonly used property to classify the micro-valve systems. For multi channel applications, micro-valve arrays are used. With micro-valve arrays, more precise flow is achievable since flow regulation is easier with an array than one valve. Another advantage of micro-valve arrays is they also have a relatively low cost.
Micro-valves may be micro-machined from silicon; however, other materials also may be used. Silicon has many advantages for MEMS (micro-electrical mechanical system) applications. It is one of very few materials that can be economically manufactured in single crystal substrates. Its crystalline nature provides significant electrical and mechanical advantages. Besides, silicon is abundant and can be produced in high purity and is an elastic and robust material. Good sealing properties make silicon the most used material for micro-valve applications. In particular, spin-on silicone rubber is very attractive for micro-valve applications since it has low modulus, high elongation, good compatibility with IC processes, and good sealing properties. Glass, polymers, and thin metal films such as Ni, Ti, Fe, and Cu may also used in micro-valve fabrication.
The use of micro-valves has enabled more precise operation of the control systems for HVAC systems. However, difficulty of controlling the superheat in HVAC systems continues to be experienced as thermal load on the HVAC system changes due to changing environmental conditions. Accordingly, it would be desirable to provide a stable control algorithm for micro-valves in HVAC systems that would compensate for thermal load changes.